Channel handlers should be more selective in the offer they make

Current situation

Channel handlers (agents & ssh) make offers strictly based on tags and namespaces. They do not attempt to make a finer offer: if a job requires a,b and channels are available offering a,b and a,b,c, they may offer a,b,c, which would prevent a more requiring job to receive an offer immediately.

Desired outcome

Channel handlers should make more selective offers (offering a,b in the example above, for example).